1. Field of the Invention
This invention relates to piping corrosion monitoring systems. More particularly, it relates to a monitoring system that generates an inspection schedule from specified input data describing the piping system.
2. Related Art
All of the metals commonly used in piping or associated vessels in process plants, such as oil refineries, chemical processing facilities, pharmaceutical manufacturing plants, and other industrial environments, corrode with time. The rate of corrosion of a particular pipe or vessel is affected by the type of metal employed, the substance contacting the metal, the pressure and temperature within the pipe or vessel, and other complex factors. Failure of the piping or associated vessels can be expensive and dangerous, and even catastrophic. Accordingly, these and other industries have recognized for many years the necessity for monitoring the condition of piping and vessels within process plants.
In order to limit unnecessary plant shutdowns and to avoid accidents, the condition of the piping in a plant should be periodically inspected. More particularly, the piping wall thickness should be measured from time to time to determine when individual piping elements should be retired from service, that is, to determine when the extent of corrosion has caused the pipe to reach its "retirement limit."
Typical methods of monitoring piping thickness involve acoustic monitoring using ultrasonic probes or radiographic monitoring using a radioactive source and radiosensitive film. Development of ultrasonic thickness-measuring instruments and of radiographic techniques has allowed the wall thickness of piping to be monitored while the plant is operated. This allows inspectors to locate problems before they create dangerous conditions or cause unplanned shutdowns.
Unfortunately, most piping in a refinery, for example, is very difficult to reach, and is frequently insulated. This makes it difficult to use ultrasonic probes. The practice is therefore to inspect piping only at selected locations, referred to as inspection points. Ideally, these inspection points are chosen by experienced piping inspectors to be representative of the worst-case corrosion conditions within a particular section of the refinery.
It is generally understood that piping elements exposed to the same corrosion environment, that is, to the same combination of corrosion-affecting factors, will corrode in a similar way. If groups of pipes and associated vessels and the like which are exposed to the same corrosion environment are accurately grouped into "circuits," actual corrosion measurements taken at one inspection point in a circuit can be used to infer corrosion conditions in other portions of the circuit. This information can be used to determine the expected life for individual pipe sections and to determine a reasonable inspection schedule, that is, to schedule future inspections, to determine whether or not the anticipated corrosion rates are in fact experienced in practice.
There are several factors that affect the proper design of a piping corrosion monitoring system, particularly one which is to be implemented by computer program. For example, the program must take into account that there are several distinct types of corrosion mechanisms, and several different pipe failure modes according to which actual corrosion data measured at a first inspection point in the circuit must be processed in correspondingly different ways to yield realistic predicted data and projected inspection dates for piping not specifically inspected.
For example, a pipe may begin to leak when corrosion in a particular area, e.g. a pit, reaches its outside surface. On the other hand, a pipe may also fail when a large portion of its wall has been significantly corroded, such that the pipe splits or buckles in service, thus rupturing completely. In the former case, corrosion must extend all the way through the pipe wall for failure to occur. By comparison, in the latter case it may only be necessary for 80% of the wall of the pipe to be corroded away for it to rupture.
Prior work has recognized the desirability of separating the piping and associated vessels in a process plant into circuits having essentially common corrosion environments. It has also been recognized that multiple failure modes are possible within a single circuit and that these should be treated differently in connection with establishment of an inspection schedule based on anticipated corrosion rates, particularly where implemented by computer.
For example, Buhrow, "A Complete Computer Program for Inspection of Refinery Piping" Preprint No. 09-68, presented during the 33rd Midyear Meeting of the American Petroleum Institute's Division of Refining, May 15, 1968, discusses the piping circuit concept, according to which piping or other vessels in the plant that are exposed to the same corrosion environment are treated together, so that data taken from one inspection point in the circuit can be used to project corrosion at other points in the circuit not actually inspected in a particular inspection sequence.
In Buhrow, "The Computer Assists the Refinery Inspector: When to Inspect Piping," Preprint No. 35-71, presented during the 36th Midyear Meeting of the American Petroleum Institute's Division of Refining, May 13, 1971, which is incorporated by reference herein, the circuit concept is discussed further. This paper also mentions that some circuits, for example, those that carry more dangerous substances, or which would for other reasons cause more significant damage to property or be dangerous to persons if a failure occurred, must be monitored more closely than others. However, according to this paper, such matters should simply be taken into account by the inspector in selecting the inspection points, and in setting the "safety factor" of the circuit, that is, in determining the inspection date for particular elements of piping within each circuit.
This paper also describes a further concept useful in establishing a corrosion monitoring analysis program, the concept of the "test case." For the purposes of this application a "test case" defines a manner of calculating a predicted corrosion rate, that is, the test case is a mathematical model of a corrosion mechanism, and is thus useful in modeling and predicting corrosion.
A particular test case may take into account historical data for individual points in the system, for the overall circuit, the inspector's experience, a particular corrosion mechanism modeled, and like factors. For example, one test case which is of use defines the "point long rate." This term refers to the long-term rate of corrosion of the piping in the vicinity of a particular inspection point, which may be determined by dividing the total loss of piping wall thickness by the period between the earliest and latest inspection dates. Similarly, the "point short rate" is the amount of corrosion loss experienced between the two most recent inspection dates divided by the time interval between them.
It will be appreciated that if corrosion conditions are consistent the point long rate will ordinarily substantially equal the point short rate. Therefore, if after a particular inspection the point short rate substantially exceeds the point long rate for a particular inspection point, some factor contributing to corrosion at that inspection point has evidently changed, and some further investigation may be warranted.
Another exemplary test case described in the 1971 Buhrow paper relates to the "circuit formula-adjusted average rate." According to this test case, the circuit average rate, that is, the average of all the corrosion rates measured at the inspection points within a circuit, is multiplied by a statistically significant numerical factor. This test case provides a statistically significant estimated corrosion rate, which may be compared with other corrosion rates, such as the point long rates and point short rates, to yield a "worst-case" prediction for the corrosion rate.
More particularly, the 1971 Buhrow paper also reports that corrosion rates within a process plant were found to obey Gaussian statistics. Accordingly, the corrosion rates measured within a given circuit exhibit a normal Gaussian distribution, according to which values for generally comparable measured items are clustered about an average value. Therefore standard statistical methods can be used for corrosion-rate analysis, in particular, for assignment of risk factors to various sampling techniques.
For example, as described above, the circuit average rate is the average of all corrosion rates monitored in a particular circuit divided by the number of inspection points. This rate is useful in estimating corrosion according to one of the test cases. However, where the number of measurement points is small, one's confidence in the accuracy of the calculation, that is, in any conclusion to be drawn therefrom, is low; a large sample always affords greater confidence in the accuracy of statistical data analysis than does a small sample. Accordingly, where the number of corrosion rates actually measured is small, the calculated average rate may be adjusted by addition of a factor which statistically takes into account the standard deviation of the data and the number of actual measurement points used to generate the data, thus "factoring-in" the confidence value of the data. This is discussed in connection with FIG. 6 of the 1971 Buhrow paper under the heading "The Average Rate Adjustment Formula."
This paper also discusses the "maximum/average corrosion rate ratio," which is the ratio between the maximum corrosion rate measured within a particular circuit and the circuit average rate of corrosion. Where this ratio is high, typically greater than about 4, this indicates that at least one of the points within the circuit is corroding at a significantly higher rate than the others, and thus provides an indication that the entire circuit may require special attention. This can be instructively contrasted with the case in which the standard deviation "sigma" is relatively high for a given circuit, which indicates that all or most of the corrosion rates measured for a given circuit vary substantially about the average. Thus, if a large number of corrosion rates within a circuit are clustered around the average, the maximum rate of corrosion at a single point within the circuit can vary significantly from the average without affecting sigma substantially. Calculation of the maximum/average ratio allows one to determine when this has occurred.
Finally, the 1971 Buhrow paper also discusses the inspection date ratio (IDR), which is the result of division of the sum of the circuit update corrosion allowances, that is, the sum of the wall thicknesses remaining in particular piping elements before their individual retirement limits are reached, by the sum of the differences in time between the actual inspection dates and the most recent measurement dates, multiplied by the average rate of corrosion. In essence, the IDR relates the rate of corrosion, the amount of material remaining in the pipes of the circuit, and the circuit frequency of inspection, to provide an indication of the degree of conservatism employed in the calculation of inspection dates.
The 1971 paper suggests, in its discussion of FIG. 4, that risk factors corresponding to various conditions, such as the hazardous nature of the material being carried by a particular pipe, should be set by the inspector. In an implementation of the techniques described in that paper, a default risk factor was assigned if the inspector failed to specify a risk factor.
A later paper, Buhrow, "Computer Forecasting Inspection Dates From Metal Corrosion Data," which was presented at the American Society of Mechanical Engineers, Energy Sources and Technology Conference and Exhibition in Dallas, Tex., Feb. 17-21, 1985, describes a computer program for corrosion monitoring. This paper essentially updates the 1971 paper discussed above.